In my experience, no "yes, I'm 100% straight" person has ever said they'd have sex with someone of the same sex just for fun. The only time they've said they would was if they had to. Otherwise, they'd prefer to go without! Most people I've chatted to who've said, 'oh, I'd give it a go' have considered themselves bi-curious, or heteroflexible, not wholly straight.
Frankly, if OP's husband is gay and not homoflexible or bisexual, I'm not sure why he'd unnecessarily force himself to have sex with someone he's not attracted to when he could just have a wank. It's surely not very 'pleasurable' if he's struggling to get it up and engage because he's so turned off by her body. Masturbation sounds sooooo much better to me than trying to have sex with either a woman or a man who I'm not attracted to. Like, why?
And I'm not sure why she'd want to have sex with someone who she knows is struggling to engage with her, and finds her body unappealing and undesirable, a poor substitute for a male body. Again, isn't a nice, leisurely masturbation session more pleasurable than knowing your husband is trying to 'think of England' (or rather, men) whilst doing the deed, because you're such a turn off to him??
It sounds incredibly fucked up when you break it down, honestly.